Developmental Characteristics of Miscanthus spp. that influence saccharification and biofuel potential

Abstract

Miscanthus spp. are woody, perennial and rhizomatous grasses which have potential for high growth rates. The rapid growth and high biomass yield of Miscanthus spp. with low fertilizer inputs make it a genus of interest for the production of biofuel. It is already used for heat and electricity production. Four developmental stages were detected in Miscanthus giganteus stems over the growing season in field collected material using light microscopy and data obtained from the measurement of stem heights, length of internodes and biomass yields. M. giganteus tissues attained 'maturity‘ by October-November after which, no further cell wall development was observed. These growth stages were used as a basis to study development of lignification in M. giganteus at different harvesting times. Early-harvested material of M. giganteus was observed to be immature having very low lignin content with subsequent development and increase in lignification in different cell types. At the time of maturity (October-November), the crop acquired its maximum degree of lignification. Development of tissues and lignification also differed conspicuously at different heights in the stem. Upper internodes of the stem appeared immature with lower lignin content whilst basal internodes were more mature and highly lignified. Development of tissues and lignification within the stem of M. giganteus proceeded acropetally. Saccharification yield analysed using NREL protocols fluctuated with growth stage and degree of lignification throughout the season. Saccharification yield was highest at the beginning of cell wall expansion stage (July), lower during August-September, higher at cell wall thickening stage (October) and then was lowest during maturation and senescence (November to January). The bottom internodes with thick cell walls showed higher glucose yield after saccharification than the upper internodes in which cells were still elongating and expanding. Saccharification levels and patterns in M. giganteus did not vary substantially at different geographic locations and observation from the flowering species Miscanthus sacchariflorus and Miscanthus sinensis (as well as on the main material of the study M. giganteus which is non-flowering) also showed that the seasonal saccharification yield, particularly the characteristic reduction in saccharification yield, was not due to a pseudo flowering in M. giganteus. Taken together, the results of saccharification at different harvesting time and at different heights in the stem over one growing season suggest October to be the ideal month to harvest the crop for enzymatic glucose release. Enzymatic degradation by the action of cellulase enzymes on different tissues and cell types in 27 μm transverse sections of M. giganteus stem was also studied under light microscopy from early-, mid- and late-harvested materials. Early-harvested material in July (immature and less lignified) showed the highest rates of cell wall decay, whereas late-harvested material in November and January (mature and with high degree of lignification) showed a low degree of cell wall degradation. The most readily degradable tissues were the phloem, followed by parenchyma and fibre cells. The cortex and epidermis were found to be recalcitrant to the enzymes. However, thin cross section (2 μm) of embedded material showed some evidence at high resolution of degradation of sclerenchyma, fibre cells and some cortex cells under the epidermis. Starch levels determined at different times over one growing season and at different heights in the stem showed significant influences of stem age and geographic location. Different locations influenced the concentration of starch. The starch concentration was not uniformly distributed throughout the plant and was higher in mature (bottom) internodes than younger (upper) internodes.